1. Field of the Invention
The present invention relates to a light pipe excellent in light-utilizing efficiency to make it possible to form a reflection type liquid-crystal display device bright and easy to view, and a plane light source unit using the light pipe.
The present application is based on Japanese Patent Applications No. 2000-190512 and 2000-258354, which are incorporated herein by reference.
2. Description of the Related Art
There is known a reflection type liquid-crystal display device using a front-lighting system in which a plane light source unit including a side-lighting type light pipe making viewing in a dark place, or the like, possible is disposed on the visual side and in which an anti-reflection layer including a circularly polarizer is added to a lower surface of the light pipe (Unexamined Japanese Patent Publication No. Hei. 12-111900). When a display screen is viewed through the plane light source unit disposed on the visual side, light reflected by a liquid-crystal cell, or the like, under the light pipe overlaps a display image to thereby cause white paling or lowering of contrast, resulting in deterioration of visibility both in an operating mode and in an external light mode. Therefore, the anti-reflection layer is provided for preventing the reflected light from being generated. In the background-art reflection type liquid-crystal display device, however, lowering of luminance caused by addition of the anti-reflection layer is conspicuous in an operating mode. Moreover, luminance decreases largely as the position goes farther from the light source. There is a problem that variation in brightness and darkness is large.
Further, there is also known a reflection type liquid-crystal display device using a front-lighting system in which a plane light source unit constituted by a side-lighting type light pipe to make viewing in a dark place, or the like, possible is disposed on the visual side and in which a light-diffusing layer having a surface of fine prismatic structures is added to a lower surface of the light pipe through an adhesive layer (Unexamined Japanese Patent Publication No. Hei. 11-281980). Output light (display light) interferes with light reflected by the lower surface of the light pipe and by the liquid-crystal cell, so that moire (stripe-like brightness and darkness) occurs. The light-diffusing layer is provided for the purposes of: preventing the moire by scatter reflection, scattering at the time of transmission; preventing external incident light from being reflected by the lower surface of the light pipe and from forming an image of the external incident light. The moire reduces visibility greatly because a stripe pattern of brightness and darkness moves in a viewing position to form a glare visual sense. In the background-art reflection type liquid-crystal display device, however, contrariwise to the aforementioned advantage given by the addition of the light-diffusing layer, luminance in an operating mode decreases greatly as the position goes farther from a light source. Hence, there is also a problem that the display device has the disadvantage that variation in brightness and darkness becomes large.
An object of the present invention is to develop a light pipe and a plane light source unit in which an anti-reflection layer is added to thereby suppress lowering of luminance and variation in luminance in an operating mode while white paling, lowering of contrast, or the like, deteriorating visibility because of overlapping of light reflected by the lower surface side of the light pipe with a display image is prevented both in an operating mode and in an external light mode to thereby make it possible to form a reflection type liquid-crystal display device which uses a front-lighting system and which is excellent in visibility such as contrast, brightness, etc.
An object of the present invention is to develop a light pipe and a plane light source unit in which a light-diffusing layer is added to thereby suppress variation in luminance in an operating mode while preventing an image of external incident light from being formed and preventing moire from occurring to thereby make it possible to form a reflection type liquid-crystal display device which uses a front-lighting system and which is bright and excellent in display quality.
From a first aspect, the present invention provides a reflection type liquid-crystal display device comprising a light pipe including a plate-like member having light output means formed in its upper surface so that light incident on an incidence side surface of the plate-like member is exited from the lower surface of the plate-like member through the light output means, an adhesive layer having a refractive index lower than that of the plate-like member, and an anti-reflection layer made of a circularly polarizer and bonded to a lower surface of the plate-like member through the adhesive layer; a plane light source unit including at least one light source disposed on one side surface of the light pipe; and a liquid-crystal cell including a reflection layer disposed on the light output side of the plane light source unit.
According to the above invention, the anti-reflection layer including a circularly polarizer and provided on the lower surface of the plate-like member suppresses light reflected by the lower surface of the light pipe and leaking from the upper surface of the plate-like member without entering the liquid-crystal cell or the like. Hence, the leaking light to be a cause of generation of white paling or lowering of contrast because of overlapping of the leaking light with the display image from the liquid-crystal cell in viewing from the upper surface is little so that good visibility can be achieved both in an operating mode and in an external light mode. Incidentally, the reflectance by the lower surface of the light pipe is estimated as a value of from about 3 to about 5% in light output from the light pipe or in external incident light. In this case, the reflected light has strong influence on white paling or contrast of the display image if the reflected light is not suppressed by the anti-reflection layer.
On the other hand, the anti-reflection layer is bonded through an adhesive layer having a refractive index lower than that of the plate-like member. Hence, lowering of luminance or variation in luminance in an operating mode can be reduced greatly. This is based on the investigation on the problem in the background-art method. That is, it was conceived in the background art that suppression of reflection in the interface between the plate-like member and the anti-reflection layer was favorable for improvement of luminance and that bonding the plate-like member to the anti-reflection layer through an adhesive layer having a refractive index as near as possible to reduce the difference between refractive indices in respective interfaces as greatly as possible was favorable for improvement of luminance. In this case, however, light xcex20 incident on the side surface of the plate-like member or transmitted light thereof is apt to be transmitted through the adhesive layer 14 because of the lowering of the interfacial refractive index difference and enter a linearly polarizer 12b constituting the anti-reflection layer 12 as represented by the bent arrows xcex20, xcex21 and xcex10xe2x80x3 in FIG. 8. Generally, about a half of the light xcex21 entering the linearly polarizer is absorbed. Hence, the light xcex10xe2x80x3 transmitted backward is reduced greatly because of the absorption loss of the light xcex21.
On the other hand, in accordance with the present invention, the incident light xcex20 is apt to be totally reflected because of the refractive index difference between the plate-like member and the adhesive layer 13 as represented by the bent arrows xcex20 and xcex10xe2x80x2 in FIG. 7. Moreover, light which is large in incident angle on the adhesive layer to be apt to be transmitted backward is influenced easily by the total reflection. Hence, the light hardly enters the linearly polarizer and the efficiency of backward transmission of light is improved. As a result, luminance is improved and variation in luminance is reduced, so that uniformity of luminance in the light exit surface of the light pipe is improved. Hence, a plane light source unit for a front-lighting system can be obtained as a unit excellent in light-utilizing efficiency and excellent in brightness and its uniformity. Hence, a reflection type liquid-crystal display device using a front-lighting system can be obtained as a device excellent in contrast and brightness both in an operating mode and in an external light mode and excellent in display quality.
From a second aspect, the present invention provides a reflection type liquid-crystal display device comprising a light pipe including a plate-like member having light output means formed in its upper surface so that light incident on an incidence side surface of the plate-like member is exited from the lower surface of the plate-like member through the light output means, an adhesive layer having a refractive index lower than that of the plate-like member, and a light-diffusing layer having a surface of fine prismatic structures and bonded to the lower surface of the plate-like member through the adhesive layer; a plane light source unit including at least one light source disposed on one side surface of the light pipe; and a liquid-crystal cell including a reflection layer disposed on the light output side of the plane light source unit.
According to the above invention, the light-diffusing layer is bonded through an adhesive layer having a refractive index lower than that of the plate-like member. Hence, variation in luminance in an operating mode can be suppressed greatly. This is based on the investigation on the problem in the background-art method. That is, it was conceived in the background art that suppression of reflection in the interface between the plate-like member and the light-diffusing layer was favorable for improvement of luminance in terms of prevention of reflection loss, and that bonding the plate-like member to the light-diffusing layer through an adhesive layer having a refractive index as near as possible to reduce the difference between refractive indices in respective interfaces as greatly as possible was favorable for improvement of luminance in terms of suppression of reflection loss.
In this case, however, light xcex20 incident on the side surface of the plate-like member 101 or transmitted light thereof is apt to be transmitted through the adhesive layer 115 because of the lowering of the interfacial refractive index difference and enter the light-diffusing layer 112 as represented by the bent arrows xcex20, xcex20xe2x80x2, xcex21xe2x80x2 and xcex21xe2x80x3 in FIG. 15. The incident light is scattered xcex21xe2x80x2 in the interface between the fine structure surface and air, so that there is generated a great amount of light xcex20xe2x80x2, xcex21xe2x80x3 leaking from the upper and lower surfaces of the light pipe. In addition to this, the light xcex20xe2x80x2 leaking from the lower surface is made incident on the liquid-crystal cell 120 at a large incident angle. Hence, the light leaking from the upper and lower surfaces does not contribute to illumination of the liquid-crystal cell and reduces the light to be transmitted to the back of the plate-like member. Moreover, the light xcex21xe2x80x3 leaking from the upper surface is apt to cause lowering of contrast of display light. In addition, scattering owing to the light-diffusing layer brings variation also in the directions of the output light xcex21, xcex22, xcex23 and xcex24 serving as illumination light or display light through the light output means, or the like, to thereby deteriorate light-condensing characteristic to thereby darken display in the viewing direction because of the enlargement of the light distribution.
On the other hand, in accordance with the present invention, the incident light xcex10 is apt to be totally reflected because of the refractive index difference between the plate-like member and the adhesive layer 113 as represented by the bent arrows xcex10 and xcex11 in FIG. 14. Moreover, light which is large in incident angle on the adhesive layer to be transmitted backward is influenced easily by the total reflection. Hence, the light hardly enters the light-diffusing layer 112 and the efficiency of backward transmission of light is improved. Moreover, the directions of the output light xcex11, xcex12, xcex13 and xcex14 through the light output means, or the like, hardly vary. Hence, light-condensing characteristic is improved to thereby improve luminance in a narrow light distribution to there by make display bright in the viewing direction. As a result, a plane light source unit for a front-lighting system can be obtained as a unit excellent in light-utilizing efficiency and excellent in brightness and its uniformity. Hence, a reflection type liquid-crystal display device using a front-lighting system can be obtained as a device excellent in brightness and display quality both in an operating mode and in an external light mode because moire or an image of reflected light of external light is hardly generated.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.